Polylactic acid is a plant-derived biomass polymer, and has been receiving attention as a resin alternative to petroleum-derived polymers. Polylactic acid, which is a highly elastic and strong polymer, unfortunately, lacks toughness and has low impact resistance, low tear resistance, and low flexibility. Polylactic acid has a low rate of crystallization, and barely shows crystal growth in ordinary crystal growth. Although the melting point is approximately 170° C., polylactic acid thermally deforms at temperatures of not less than the glass transition temperature, i.e., not less than 60° C., and cannot keep a film shape. Then, to improve the heat resistance of polylactic acid resin films, several methods have been heretofore suggested.
As the measures against these problems, a method of blending polylactic acid with a soft and heat resistant polymer to improve the heat resistance of a polylactic acid resin film has been suggested (PTL 1).
Alternatively, a method of adding aliphatic polyester/core-shell type rubber to polylactic acid, and monoaxially or biaxially drawing a film formed of the prepared polylactic acid has been suggested (PTL 2). Both methods can attain impact resistance and heat resistance at the same time. Unfortunately, blending of large amounts of the petroleum-derived polymer and additives significantly reduces the ratio of the plant-derived component (degree of biomass).
A technique of giving flexibility and heat resistance to a polylactic acid film has been suggested, in which crystallization of a resin composition comprising polylactic acid, a plasticizer, and a nucleator is promoted in a heat treatment step subsequent to a step of molding a film (PTL 3). Unfortunately, in this method, addition of the plasticizer may cause bleed-out, and an effect of improving tear resistance is little while an effect of improving flexibility is attained. If such a film or sheet is used as a substrate for pressure-sensitive adhesive tapes or the like, such pressure-sensitive adhesive tapes or the like may be broken or torn during the production or processing thereof.